1. Field of the Invention
The present invention relates to PWM control method and device used in a vehicle-mounted electronic control unit and a light adjusting device.
2. Description of the Related Art
Conventionally, a PWM (Pulse Width Modulation) control has been widely known as means for dimming a lamp and adjusting a motor speed in a vehicle such as an automobile or a commercial vehicle (see, for example, Japanese Unexamined Patent Publication No. 2000-177480).
For example, in an electronic control unit (ECU) mounted in a vehicle such as an automobile, a microcomputer having a PWM output function is incorporated and a PWM control signal is generated and outputted for lamp dimming and motor speed adjustment control using a PWM signal outputted from the microcomputer. The ECU changes a duty ratio of the PWM control signal by changing a duty ratio (ratio of a high-level period to one cycle) of the PWM signal from the microcomputer, whereby the lamp dimming and motor speed adjustment of the vehicle can be performed with high accuracy. In the case of PWM controlling the lamp dimming of the vehicle, the lamp can be gradually turned up by gradually increasing the duty ratio of the PWM control signal and can be, conversely, gradually turned down by gradually decreasing the duty ratio as shown in FIG. 8.
As described above, in the case of PWM controlling the lamp dimming of the vehicle, the lamp is turned on or off by changing the duty ratio of the PWM control signal. On the other hand, in order to keep the lamp on with a specified luminance, the duty ratio of the PWM control signal needs to be maintained at a specified value. Here, if the frequency of the PWM control signal is excessively low as shown in FIG. 9(a), there is a problem in the case of a vehicle interior lamp or other cases that the flickering of lamp light attracts a driver's or passenger's attention and the driver or passenger visually recognizes the flickering.
Thus, in the case of controlling the light adjustment of the lamp such as the vehicle interior lamp, it is effective in light of reducing the flickering of the lamp light to more frequently switch the level of the PWM control signal between high level and low level by increasing the frequency of the PWM control signal as shown in FIG. 9(b).
It has been confirmed that noise is created in the ECU at the time of a PWM control if an AM radio is installed near the ECU having such a PWM output function as described above, and its causes include sudden current increase and decrease occurring in the ECU. In other words, in the ECU, a PWM signal from the microcomputer flows in a circuit in the ECU to generate a PWM control signal used for lamp dimming and the like. Due to the rectangular waveform of the PWM signal, the current generated in the circuit suddenly increases and decreases at the time of generating the PWM control signal. Such sudden increase and decrease of the current lead to an increase of high frequency components of the PWM control signal, with the result that noise is induced in the AM radio installed near the ECU.
Thus, generally, the above sudden increase and decrease of the current are moderated by dampening the rectangular waveform of the PWM signal from the microcomputer, whereby the increase of the high frequency components of the PWM control signal resulting from the sudden increase and decrease of the current is suppressed.
An object of the present invention is to provide technology capable of effectively suppressing noise generation in a PWM control as described above.